1. Field of the Invention
This invention relates to the production of a jet fuel range olefinic distillate and, more specifically, to the co-processing of light olefins with jet fuel range distillate over a large pore zeolite to result in the formation of a substantially aromatic-free jet fuel range distillate and a high molecular weight alkyl aromatic fraction.
Jet fuels are petroleum stocks that boil in the range of about 350.degree. F. to about 700.degree. F. with the major portion boiling below about 500.degree. F. The performance of jet fuels depends on the cleanliness of combustion which is measured by the smoke point of the fuel and, of course, will depend on the heat value of the fuel. One limit to jet fuel quality is the aromatic content. Aromatic carbons increase smoke emissions and generally decrease the heat value of the jet fuel.
At present, removal of aromatics from jet fuel quality distillate is accomplished by solvent extraction to strip the aromatics or by severe hydrotreating of the distillate to saturate the aromatic carbons. Various solvents are presently employed in solvent extracting the aromatics from petroleum distillates. Furfural is one example of a versatile solvent used for extracting aromatics from all petroleum stocks higher-boiling than gaoline. Hydrotreating to saturate aromatic carbons and improve the jet fuel quality requires severe process conditions such as very high hydrogen partial pressures (greater than 1,500 psi) to obtain high saturation at reasonable reaction temperatures and contact times.
The present invention is concerned with a process of upgrading jet fuel quality by altering the aromatic materials contained within the jet fuel distillate. Aromatic content of the jet fuel is altered by co-processing the jet fuel range distillate with light olefins over large pore zeolites. Upon reaction, much of the aromatic content of the jet fuel range distillate is converted to high molecular weight alkyl aromatics, thus shifting many of the aromatic carbons in the 300.degree. F. to 500.degree. F. fraction into the 550.degree. F.+ range. The alkyl aromatics can then be removed from the olefinic jet fuel by distillation.
2. Description of the Prior Art
The production of alkyl aromatic compounds by reacting aromatic and olefinic hydrocarbons is well known. For example, U.S. Pat. No. 2,904,607 relates to a process for the production of alkyl aromatic hydrocarbon compounds of high anti-knock value, which are of suitable boiling range for use as motor fuels. This patent discloses that aromatic hydrocarbons may be particularly readily alkylated with olefins by contacting the reagents at moderately elevated temperatures with a crystalline aluminosilicate catalyst having pore openings adequate to admit freely the individual aromatic and olefinic molecules, the pore openings being about 6 to 15 Angstroms.
U.S. Pat. No. 3,251,897 also relates to the alkylation of aromatic hydrocarbons in the presence of an aluminosilicate catalyst. The alkylating agents may be employed in fluid media which contain major proportions of inert diluents.
U.S. Pat. Nos. 4,301,316 and 4,301,317 disclose processes for the selective alkylation of aromatic compounds with a relatively long chain length alkylating agent to produce linear phenylalkanes enriched in the 2-phenylalkane isomer. The process utilizes a selective zeolite catalyst characterized by a crystal structure having channels or network of pores therethrough, the major dimension of the openings to said channels or networks of pores being about 6 and about 7 Angstroms.
U.S. Pat. No. 3,663,428 relates to the co-processing of lower olefins and a tar fraction consisting essentially of polycyclic aromatic compounds to obtain a product comprising a mixture of compounds having alkyl side chains. The tar fraction which is employed is obtained from the heavy cracking oil produced by thermally cracking petroleum hydrocarbons at a temperature above 700.degree. C. and below 2300.degree. C., and subsequently removing solid pitch from the heavy cracked oil, and which consists of a fraction boiling at 200.degree. C. to about 500.degree. C., calculated at normal pressure. The alkylation reaction is conducted by mixing the gaseous olefin with the tar fraction and passing the mixture over a solid catalyst bed. The catalyst to be used includes silica-alumina catalysts among others. The alkylated tar produced by the process of the patent is used as an electric insulating oil, a rubber processing oil, a heat transfer oil, a plasticizer, etc.
U.S. Pat. No. 3,894,939 discloses co-processing a gasoline boiling range fraction which contains about at least 20 weight percent aromatics and a gas oil fraction having an end point up to about 850.degree. F. and a pour point of at least about +20.degree. F. over a zeolite catalyst to form a product comprising a gas oil fraction having a lower pour point, a gasoline fraction having a higher octane and a light gas fraction.